Printed circuits are formed with major dimensions of length and width and contain one or more circuits. The thickness of the printed circuit panels varies for many different reasons and directly effects panel flexibility.
For example, printed circuits made with multiple conductor planes use a bonded dielectric layer or layers for separation. Holes perforated through the circuit substrate serve a number of purposes including solder terminals for installation into another assembly, plated through hole interconnections between conductor planes, and tooling registration holes.
Exposed copper on the finished circuits must, with few exceptions, be solder coated, often termed presoldering. It is also preferable that the solder be applied only where needed later and not on all conductor runs. To apply solder selectively, a dielectric, referred to as a cover dielectric or solder mask, is used to cover copper that need not be solder coated. The desired copper portions remain exposed and are solder coated, at terminal pads and the like. In other words the only exposed copper on the surface of the printed circuit is solder coated.
It is also necessary that all holes through the printed circuit be lined with solder but unobstructed by solder when finished so as not to obstruct later insertion requirements. The process of clearing the holes of solder is referred to as leveling.
More particularly, printed circuit panels are presoldered, inter alia, to maintain solderability for subsequent operations. For economic purposes, such presoldering must be accomplished as a mass panel coating technique. Known solder coating techniques include electrolytic plating, and electrolytic plating followed by hot reflow. The hot reflow may be accomplished by the use of infrared reflow techniques, hot liquid reflow or hot vapor reflow. Another mass solder coating technique is hot roller coating. Finally, hot immersion dip and hot immersion dip followed by hot leveling are also known techniques. In the latter, leveling may be accomplished by hot liquid leveling, hot vapor leveling, or hot gas leveling.
The last technique described, immersion dip followed by hot leveling, has been found to be most effective for many printed circuit applications.
The machines presently available for selective solder coating and hot air leveling apply solder by vertically dipping panels into hot molten solder. Subjecting the panels to a hot air blast levels or removes excess solder and clears all holes as the panels are withdrawn from the liquid solder. With these machines, all operations must either be done manually or linked to an up-down and horizontal index mechanism. This precludes the continuous feed of panels, a process that is economically beneficial.
Moreover, solder coating in the presence of oxygen tends to be a dirty operation due to the formation of oxides. This complicates the mechanisms of transfer of the panels.
Another problem encountered with available soldering machines results from the nature of the materials used in printed circuits which, except for the metal conductors, are sensitive to heat and thermal shock. Precise control of time and temperature in processing operations thus becomes critical. Further, vertical dipping of the printed circuit panels into a solder bath requires that the first area into the hot solder is the last to leave. With the thin flexible substrates often used in printed circuits, rapid entry into the dense liquid solder causes bending and folding, and damages the panel and the effectiveness of subsequent hot air leveling. For these reasons, slow entry of the panels into the molten solder, use of special fixturing, and wide variations of time-temperature exposure across printed circuit panels, all undersirable, are encountered.
One technique of selectively soldering and leveling printed circuits is described in U.S. Pat. No. 4,315,042 for Solder Removal Technique. The process and apparatus there described are subject to many of the disadvantages discussed above. In particular, it is not possible to provide a continuous use of the process to render it efficient and economical.
Another technique of applying solder to printed circuits uses so-called wave soldering machines. The printed circuit is passed over the solder wave which results in the application of solder to one side of the printed circuit. This technique has the disadvantage of requiring two passes to solder both sides of many printed circuit configurations.